Using .htaccess to hide the location and extension of .php file

I’m trying to have all requests going to //my.domain/download?id=xx (be it http or https) be processed by /path/to/my/download.php?id=xx. I’ve tried all sorts of RewriteRule rules to no avail, so there’s no point in me posting anything I have, but this is a WordPress install, so this shouldn’t conflict with WP’s existing rules, which are: <IfModule…